Photoelectric inspection and sorting machines



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P. NASH Sept. 24, 1963 PHOTOELECTRIC INSPECTION AND SORTING MACHINESFiled April 14, 1959 7 Sheets-Sheet 3 INVENTCIR. 'F'AUL N amp-380N52-PDA PDQ mmc m\u ooom OP OO 00667220 KNEILE mm N km ASH um ATTORNEY.

Sept. 24, 1963 P. NASH 3,105,151

PHOTOELECTRIC INSPECTION AND SORTING MACHINES Filed April 14, 1959 7Sheets-Sheet 4 INVENTEIRQ PAUL NASH.

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Sept. 24, 1963 P. NASH PHOTOELECTRIC INSPECTION AND SORTING MACHINESSept. 24, 1963 P. NASH 3,105,151

PHOTOELECTRIC INSPECTION AND SORTING MACHINES Filed April 14, 1959 7Sheets-Sheet 6 P. NASH se tf24, 1963 PHOTOELECTRIC INSPECTION ANDSORTING MACHINES Filed April 14, 1959 T Sheets-Sheet 7 ON m; 0.

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PS3 can United States Patent This invention relates to new and usefulimprovements in photo electric apparatus for inspecting sheet materialsand the method of examining sheet materials for the detection andseparation of defective parts thereof and is a continuation-in-part ofmy application Serial No. 726,- 920, filed April 7, 1958, and nowabandoned.

The present invention is more particularly useful in the manufacture ofhigh quality paper. When associated with a paper cutting machine, thedefects of the paper are detected on the uncut and continuously movingpaper sheet as it enters the cutting machine. The paper is then cut intosheets of the required size and with the aid of a deflector or gradingswitch, actuated by the output of photoelectric means and associatedelectrical devices, is sorted into two compartments. In one compartmentonly the first grade sheets without defects are stacked, while in theother compartment the defective, second grade sheets are stacked.

Defects in first grade paper consist of dark discolorations, starchspots, holes, folds, tears and similar inclusions and discontinuities.Defects may also consist of intensely small spots or larger areas offaint discolorations and may extend in the direction of the travel ofthe paper or at right angles to the direction of the travel of thepaper. The photoelectric means and associated circuits must thereforehave different characteristics for the reliable detection of defects.

In contradistinction to prior proposals, the present invention willdetect and grade all types of discolorations. The prior art disclosesrotating, fast moving scanning means or electronic flying spot scanningtype cathode ray tube for scanning and detecting the light intensity ofelementary areas of a travelling paper sheet. In such scanning methods,the photoelectric means is modulated by the scanning device itself, suchmodulation being prescut when the paper sheet under observation iswithout defects. As the said device scans the elementary areas from thestart of the selected paper section to it end, the intensity of thereflected light picked up by the photoelectric means changes. In orderto eliminate such unwanted modulation of the photoelectric means,filters must be employed which do not pass this basic scanning frequencycomponent of the signal. When the paper sheet is discolored the entirelength of the scanned paper section, the defect signals produced havethe same frequency content as the scanning frequency and therefore thedefeet in the paper can not be detected.

Scanning means or methods may be employed wherein modulation of thephototelectric means due to the scanning action itself is not present.For example, adjacent elementary areas of the sheet paper can be scannedsimultaneously by two scanning devices in conjunction with twophotoelectric pickup means with their outputs connected in push-pull,each scanning device being associated with one of the two photoelectricmeans. The combination of such a double, push-pull scanning system doesnot respond to light intensity changes which are in phase. Such systemshowever cannot detect discolorations which extend in the sheet in thedirection of the travel of the paper and at right angles to the scanningmotion. In the foregoing description, it was assumed that the elementaryareas scanned simultaneously are adjacent in the direction of movementor travel of the "ice paper. When the simultaneously scanned elementaryareas are adjacent in any other direction relative to the direction ofthe travel of the paper, then faint defects which extend in the samedirection as the adjacent elementary areas cannot be detected. Forexample, the elementary areas may be adjacent in the direction ofscanned motion and at right angles to the direction of the travel of thepaper.

Further, some high grade papers, such as for example bond paper, havenon-uniform textures. When such paper is scanned by the illumination ofan elementary area, the photoelectric means is modulated according tothe intensity changes of the bond paper texture. Such modulation istermed the noise level" of the particular paper. Investigations on bondpaper samples have shown that paper noise can result in a percentagemodulation of the photoelectric means as high as three parts in onehundred. As a result, faint defects extending over large areas of bondtype paper sheets and detracting from the quality of the paper, cannotbe detected against the paper noise level.

According to the objects of this invention all types of discolorationsand defects in sheet paper are reliably detected, the noise level of thepaper is reduced as ob served by the photoelectric means which arepositioned close to the paper in'a line and preferably at right anglesto the direction of motion of the paper sheet and such photoelectricmeans are not associated with mechanical or electronic scanning means.

For purposes of simplicity in description, the material under inspectionis paper, the invention however is applicable to the sorting of othertypes of sheet materials uncut or already cut and of substantiallyuniform reflection characteristics when unblemished. The invention isalso applicable to rewinding machines, machines which manufacture thesheet materials, sorting machines and cutting machines.

When the present invention is applied to cutting machines, the sheetpaper is fed from a generally large roll via auxiliary rollers whicheliminate flutter and folds in the material moving to an inspectionsurface or roller above which the travelling sheet is examined byphotoelectric means. The 'line of photoelectric means may consist of anumber of photoelectric cells placed side by side in such a manner thateach photoelectric cell observes a narrow strip of the paper through anaperture. For example, the strip of paper observed is of an inch in thedirection of movement of the paper and is 1.5 inches at right angles tothe direction of movement of the paper. The paper sections observed byadjacent photoelectric cells overlap so that the row of photoelectriccells views and inspects the entire width of the paper sheet. The paperis flooded by strong light, for example by two D.C. fluorescent lampsplaced close to the inspected section of the paper. The axes of suchtube shaped lamps are arranged parallel to the line of photoelectriccells. The said photoelectric cells are enclosed in and supported by alight-tight housing the lower side of Which is provided with an apertureforming structure. The construction and arrangement of the aperture issuch that only the diflused light reflected from the inspected paper canreach the photoelectric cells. The fluorescent lamps are also surroundedby a housing which serves as a light shield against externalillumination and light intensity changes of the inspected paper sectiondue .to changes in external illumination which could simulate defectsand cause rejection of non-defective sheets.

One or more of the photoelectric cells, for example a group of 10 cells,feed a common cathode follower in adder type circuit arrangement. Lightintensity changes due to defects in the paper modulate the outputcurrent of one or more photoelectric cells of the group. Such 3modulation appears as a voltage variation '(defect signal) at the outputof the common cathode follower stage. Such defect signal is amplified ina constant-gain amphfier, the output of which operates via two channelsa pulse producing stage, one for example, a Schmitt trigger stage, whichis well known in the art.

The first of the two said channels is a direct link between theamplifier and the trigger stage and uses the signal amplitude levels,proportional to' the modulation of the photoelectric cells, for theinitiation of the output pulse. The second channel consists of one ormore integrating stages the output of which can build up to a criticalvoltage level which can also initiate the trigger action,

The two said channels are used for the detection of faint defects in thepaper. When a faint defect extends at right angles to the direction ofmovement of the paper, the first channel is effective because thepercentage of modulation of the photoelectric cell increases with thesize of the defect and a signal is produced which is above the noiselevel of the paper.

When a faint defect on the paper is in the direction of movement of thepaper, the percentage of modulation of the photoelectric cell is low andsubstantially constant and is not proportional to the extent of thedefect. Such a weak unidirectional signal can be built up to therequired voltage level by the integrating stage of the second channelwhich will operate the trigger stage, even though such unidirectionalsignal is of the order of the paper background noise. In one embodimentof the invention the noise signal being of alternating polarity does notraise the output of the integrating stage. Several integrating stageswith various time constants can be used to respond to defects of varioussizes and intensities.

After a required delay, the output trigger pulse operates the gradingswitch by means of relays and actuators, the grading-switch deflectingthe cut and defective sheets into a second grade compartment. Therequired delay between the instant a defect is detected and the instantthe grading-switch must operate is determined by the construction of thepaper cutting machine. Having passed the inspection roller, the papersheet usually travels between draW-rolls, past a circular cutter andthen by means of carrier tapes moves to the grading-switch. Thus thedelay is defined by the characteristics of the particular machine towhich the photoelectric inspecting means and the sorting apparatus isattached.

Further, the percentage modulation of the photoelectric cells associatedwith one amplifier is low when a full-black diameter defect passes underthe inspection head. Interfering disturbances to the system which canmodulate the photoelectric cell currents by only a fraction of onepercent must therefore be avoided.

The embodiments of my invention as hereinafter disclosed serve theelimination of the said disturbances and enhance the reliability of thesubject inspection or sorting machine. In particular there are twocritical requirements of the inspection head and the surface above whichthe inspection takes place which must be satisfied in order to avoid theproduction of interfering signals which can simulate defects which arenot present on the sheet material inspected.

, When inspection takes place above the inspection roller, esscentricityof the roller increases or decreases the diffused reflected lightreaching the photoelectric cells throughthe aforementioned aperture.When fluorescent lamps are employed, such lamps can flicker oroscillate, even when the DC. supply potentials are highly regulated.

In order to substantially reduce modulation of the photoelectric cellsdue to the said effects, one or more cells are connected to one commonload resistor or impedance and the same number of photoelectric cells(one or more) are connected to another common load resistor orimpedance. The value of the load resistors are equal. The two sets ofphotoelectric cells are matched sistors, capacitors and coils) at theinput or output of i the cathode or anode followers, can beadjustedrelative to each other to give identical in-phase outputvoltages.

The amplifying stages of the push-pull amplifier consists preferably ofcathode coupled, longtailed pairs of amplifying means which are wellknown in the art. In such amplifiers in-phase signals simultaneouslyreaching the two said control grids are not amplified, while the signalreaching only one of the two control grids or anti-phase signalsreaching both grids are amplified. Signals due to the eccentricity ofthe inspection roller or due to intensity changes of the lamps arein-phase signals and are not amplified. Signals due to defects of thesheet material are not in-phase signals and therefore are amplified.Well balanced cathode coupled long-tailed amplifiers differentiate asmuch as 10,000 to 1 between in-phase and anti-phase signals.

The cathode coupled long-tailed pair type amplifiers will convert thesignal reaching only one grid into two symmetrical anti-phase push-pullsignals at the two control grids and plates of each amplifying stage.

Further, the light source can extinguish momentarily, but such anamplifier will not respond even to such a major disturbance in thesystem.

According to the invention, the row of photoelectric cells in theinspection head are paired and connected in such a manner thatsubstantially all defects of the sheet material 'are detected, whileinterference signals being of the same or greater magnitude than thedefect signals, are not amplified. V Having regard to the foregoing andother objects and advantages which will become apparent as thedescription proceeds and the details become known, the inventionconsists essentially of the novel combination and arrangement of partshereinafter described in more particular detail and illustrated in theaccompanying drawings in which:

FIG. 1 is a diagrammatic illustration of the present invention which isshown in association with a paper cutting machine.

FIG. 2 is an enlarged diagrammatic view showing the photoelectricinspecting means together with the paper cutting and grading means.

FIG. 3 is a section taken on the line 33 of FIG. 2.

FIG. 4 is a schematic representation of a circuit arrangement for thephotoelectric cells and in block diagram form, of the double channelfeature of the invention, as used between the photoelectric cellamplifier and the output pulse generator.

FIG. 5 is the circuit diagram of an'integrating stage as used betweenthe photoelectric cell amplifier and the output pulse generator.

FIG. 6 is a circuit diagram of a thyratron controlled solenoid actuatoras used between the pulse generator and the grading switch.

FIG. 7 is a schematic view of the rotary cutting blade and operatingswitches shown mounted on a disk sup port.

FIG. 8 is a block diagram representation of photoelec tric cellsconnected to two amplifiers.

FIG. 9 is a block diagram representation of photoelec tric cellsconnected to more than two amplifiers.

FIG. 10 is a circuit diagram of the amplifying system and of the controlcircuit common to all amplifiers.

FIG. 11 represents waveforms of the integrator channel of the amplifyingsystem.

Referring now to the accompanying drawings wherein the present inventionis illustrated and wherein like numerals and characters of referencedesignate corresponding parts in the various illustrations, the numeral1 indicates a roll upon which the continuous paper sheet 2 is wound andstored. As observed in FIG. .1, the sheet paper moves over and underidling feed rollers 3 and 4 respectively and then, after moving overinspection roller 5, passes between draw-rolls 11 and 12 to the cuttingstation where stationary cutting blade :13 and rotary cutting blade 14are located. After the sheets of paper are out, they are moved bycarrying tapes 15 to a grading switch 16 which deflects the defectivesheets onto a layboy in the second-grade compartment 19 while the sheets:of paper without blemishes or defects pass over grading switch .16 andare carried by tapes 13 onto a layboy in the first'grade compartmentwhich is indicated by the numeral 20.

In structure, inspection housing 9 is elongated as illustrated in FIG. 3and has mounted therein multiple photoelectric cells 6 and cathode oranode follower thermionic tubes 21 as seen in FIG. 2. Directly beneaththe photoelectric cells, the lower wall of the housing is formed with anarrow aperture 7 which extends from end to end of the housing. Walls 7aextend downwardly from aperture 7 and terminate in the form. of aperture7 b which also extends the full length of the housing and forms animportant feature of the invention. Additionally, the lower outer wallsof housing 9 extend downwardly from the body of the housing and providespaced, individual compartments within which fluorescent lamps 8 aremounted :on opposite sides of aperture 7b all of which will be laterreferred to.

By again referring to FIG. 1 it will be seen that idler roller 4 is sopositioned that by being close to'yet below the level of inspectionroller 5', it eliminates flutter and folds in paper sheet 2 and directsthe paper in such a manner that it follows the curvature of asubstantial segment of the inspection unit 5. Thus, lamps 8 can bepositioned close to the surface of the paper and will not interfere withthe optimal position of aperture 7. The draw rollers 11 and 12 are alsoso positioned that the latter purpose is served.

The rotating inspection roller 5 is not an essential feature of thepresent invention since a similar stationary, polished surface canreplace the rotatable inspection roller. The former type of inspectionroller has the advantage of minimum friction between the moving paperand the surface above which the inspection of the paper takes place butit also has the disadvantage of causing error signals when the axialsymmetry of the roller is imperfect.

A non-rotating polished inspection surface is free from theaforementioned disadvantages, however, friction between the paper andthe inspection surface can build up high static charges upon the paper,but such static electricity can be eliminated by the use of radiatingisotopes preferably located below the inspection surface.

The inspection surface of the member 5 is preferably dark andnon-reiiecting in order to detect the transparent defects or shiners,and such surface is well suited for certain types of relatively heavy,white and light coloured papers. When li ht weight and transparentsheets are being inspected, a white, polished metallic or gray surfacewith high reflection co-efiicient will minimize the lack of uniformityof the unblemished paper texture due to transparency, while thedetection of shiners is still possible. Various grades of paper ofvarious thickness and colour require different coloured light or darkerinspection surfaces in order to detect, with optimal efficiency, alltypes of defects and in order to minimize the inherent paper backgroundnoise. The inspection mem- 6 bers 5 can be made removable in order togive best performance with various grades of paper.

The photoelectric inspection means consists of a number of photoelectriccells one of which is indicated by the numeral 6 in FIGS. 1 and 2.Aperture 7 is formed between parts of the housing 9 and 10 while lampsf: illuminate the paper sheet 2. The lower section 10 of the housing ispositioned close to the paper in order to exclude external light, whilethe light illumination of the inspected section of the paper is keptconstant in the frequency response range of the amplifying system towhich the photo electric cells 6 are connected, otherwise, changes ofexternal illumination or of the light output of lamps 8 could initiatefalse defect signals which would result in deflecting unblemished papersheets into the second grade compartment. The lamps 8 are preferablyD.C. fluorescent lamps which have the form of elongated tubes whichextend the entire width of the paper 2 and such lamps are fed from aregulated D.C. power supply. Regulation and peak to peak ripple of suchsupplies is better than a 0.5 part in a hundred for plus and a minus 10part in a hundred line input voltage variation.

In FIG. 2, I have illustrated a cathode or anode follower therrnionictube 21 which is fed by an optimum number of photoelectric cells 6. Theline of photoelectric cells illustrated in FIG. 3 shows them close toone another for viewing the entire width of the inspected sheet. As anexample, if there are sixty photoelectric cells, groups of ten cells maybe joined to one cathode follower 21 so that there will be six cathodefollowers mounted inside housing 9 and each cathode will be connected toa separate amplifier.

Connections between the cathode followers 21 and associated amplifiersmay take the form of shielded cable members 22 and the amplifiers may bepositioned any convenient distance from the photoelectric means. Theshielded cables carry the cathode follower signal output lead as well asleads for the D.C. supply to the cathode follower and the photoelectriccells. As previously mentioned, the photoelectric pickup means hereinconsists of a row of photoelectric cells which Will reliably detect allblemishes present on paper sheets which mave substantially uniformreflection characteristics. Upon investigation of various types of highgrade bond paper, it has been shown that if such paper is scanned by theillumination of an elementary area 0.25 inch x 0.25 inch equals ,6 of asquare inch, the percentage modu-' lation of the reflected, diffusedlight can be as high as three parts in one-hundred. When the illuminatedelementary area is reduced, the modulation due to the unblemished papertexture usually increases and the light intensity is reduced. Thus, forpurposes of the following consideration, it is reasonable to assume apractical elementary spot size of of a square inch area.

When it is required to detect a small black spot of say inch x inchdimensions, that represents an area of approximately A of a square inch.Therefore, the percentage modulation, that is the defect signal,obtained when a blemish of of a square inch area isscanned by anilluminated elementary area of of a square inch is 1.6 parts ofone-hundred so that the defect signal is below the noise level of thepaper (three percent) and can not be detected. For reliable detection,the defect signal should be at least twice the value of the noisesignal.

Further investigations on a wide selection of bond type papers haveshown that when a narrow section, say & of an inch wide by 1.6 incheslong is illuminated, the noise modulation is a maximum of one part inone-hundred or less. When the length of such section of paper isincreased, the maximum noise modulation is reduced to less than one partin one-hundred. The reduction of noise modulation is due to theaveraging out of the modulation caused by the small size of the distinctareas of the texture having different shades or difierent reflectiveproperties.

When a black spot having a 0.001 square inch area enters an inspectionsection of X inch by 1.6 inches, which equals 0.05 square inch area,the. resulting percentage modulation is 0.001/ 0.05, that is to say, twoparts in onehundred. Therefore, such blemish is detected over the onepercent paper background noise modulation.

When the outputs of several photoelectric cells are connected to thecontrol grid of a cathode or anode follower stage, the modulation due tothe paper background noise is reduced while the modulation due to theblemishes is added. For example, it is practicable to connect tenphotoelectric cells of the IP39 type to a common anode follower.

In what follows, it is shown that the inherent noise of thephotoelectric cells due to the current passing from the photocathodes tothe anodes caused by the reflected illumination of the unblemished paperand collected by the photocathodes is by order of magnitudes smallerthan the paper background noise. Inherent noise in high vacuumphotoelectric cells, such as for example in type IP39 cells used forinvestigation, is caused mainly by the shot effeet which is the genericterm given to current fluctuations in a beam of electrons arising fromthe randomness of emission of the photocathode. The mean squarefluctuation of the photoelectric current di =leldfi wheredi=instantaneous fluctuation of current from its mean value I,e=electron charge=l.6 10- coulombs, df=frequency interval in which E isobserved. Therefore, the mean noise voltage /2eIdf-R, where R=the loadresistance of the photoelectric cell.

A inch by 1.6 inch illuminated aperture gave, in each photoelectriccell, a current 1:3.3-10-5 amperes. The load resistance R was 1 megohm.The frequency response range of the photoelectric cells, anode followerand amplifier system d =3-10 c./s. for a paper velocity of two-hundredinches per second. Therefore, in ten cells 'd 12= /2- 1.6- 10- -33 -lO--3- 10 10 /T=18 microvolts. Measurements have confirmed such theoreticalnoise levels. The highest paper background noise peak voltage vcorresponding to a modulation of one percent of I is v=1-l0- -R=33Omicrovolts This is the paper background noise voltage pickup by one cellonly, so that the inherent noise of ten type 1P39 photoelectric cellscan be neglected against the basic paper noise.

In FIG. 3, the plan view illustrates the housing 9 with aperture 7extending lengthwise from end to end thereof and the relative positionof the multiple photoelectric cells 6 with respect to the aperture 7. Itwill also be observed that the housing 9 extends the entire width of thesheet paper 2 so that in operation the entire Width of the sheet 2 issubject to inspection.

As illustrated in FIG. 4, photoelectric cells 23, 24, 25 and 26 areparalleled by connecting their cathodes to the grid 34 of triode 32. Forpurposes of simplicity only four photoelectric cells are shown. Thenumber of photoelectric cells thus paralleled may be ten or more as longas the inherent noise of such a group is below the paper backgroundnoise and as long as the paper background noise is half or less thanhalf of the signal produced by the smallest defect to be detected.Additionally, instead of the triode 32, a pentode or other type ofamplifying devices may be used. The anode follower stage consists oftube 32, anode-grid feed-back resistor 29, grid-leak resistor 30,cathode bias resistor 31, coupling capacitor 28 and anode resistor 27,and is the preferred form of coupling, but could be replaced by acathode follower stage which is well known in the art.

When the paper under inspection has a uniform texture, withoutblemishes, the amount of light reflected from the illuminated papersheet falling upon the photocathodes of cells 23, 24, 25 and 26 isconstant and the current in the photoelectric cells 6 is also constant.When however a paper defect such as a dark area is viewed by one or morephotoca-thodes, the steady current of the cells is less. Withoutteed-back resistor 29 such current decrease would produce a voltagedecrease at the grid 34 as defined by the current change and the valueof resistor 30. Due to the amplification of tube 32 and feed-backimpedance 28 and 29, substantially all the change of the photoelectriccell currents is flowing to the anode 33 and the potential of grid 34remains substantially constant so that the eflecrtive load resistance ofcells 23, 24, 25 and 26 is resistor 29.

The grid potential of the triode 3-2 remains substantially constantduring modulation hence the limiting effect upon the high frequencyresponse of the large circuit capacitance between the photocathodes andground and photocathodes and the photoanodes is substantiallyeliminated. Due to the wiring and paralleling of many photocathodes tothe common grid 34, such capacitance can be as high as micromicrofarads.

It can be shown that when the tube 32 is a pentode with an open loopgain of and when the highest frequency component of the photo-signalmodulation is 3 kilocycles, when the modulation due to a black spot of0.001 square inch area is 6.6-10- amperes, the resistor 29 can be 6.8megohms, so that the peak signal at the anode 33 is approximately 5millivolts.

Such minimum signal shown at 36 in FIG. 4 is amplitied to a 50 voltlevel by a conventional amplifier 37 in order to operate the triggergenerator 39. In order to raise the 5 mi-lli-volts minimum input signalto the required 50 volt level, amplifier 37 has a voltage gain of 10,000and is conveniently an R.C. coupled A.C. amplifier wherein the gain isconstant and is stabilized against tube changes and ageing by limitingthe stage gain of each stage of a total of four stages to ten. This isconveniently arranged by the use of anode follower type teed-backnetworks for each stage.

The outputsignal of the amplifier 38 shown in FIG. 4 initiates theoutput pulse 43 of the pulse generator 39 for which various conventionalpulse producing circuit arrangements may be employed. For example, abiased off grid controlled gas-filled triode, known in the art as athyratron can be switched into conduction by sigml 38 which has .apositive polarity or similarly, a Schmit-t trigger circuit may beemployed.

The cu-t-ofl bias applied to the control grids of the said devices issuch that the paper noise level is safely below the trigger voltagelevel so that only the defect signal will initiate the output pulsewhich, in turn, actuates the grading switch.

When the defect to be detected is a x inch square black spot or is aweaker blemish extending at right angles to the direction of travel ofthe paper, an amplitude sensitive direct input channel between amplifier37 and pulse genera-tor 39 is eflective. For example, defects which areinch wide and have the following intensities and dimensions at rightangles to the paper motion will produce identical output signals.

Size of defects in inches Intensity of defect In the At right expressedin direction of angles to fractions of paper paper full black motionmotion 3432 is: 1 %2 Ms $62 /2 is %2 it Me 362 1 3412 it: 10 $620 It ispointed out however that when a weak blemish extends in the direction oftravel of the paper and is narrow at right angles to the paper travel,integrating links are used between amplifier 37 and pulse generator 39,one such link being shown at 41 in FIG. 4. Section 45 of the signal 4!?shown in FIG. 4, is such an amplified defect signal.

Amplitude .6 of the unidirectional defect signal is smaller than thepeak amplitude '47 of the paper background noise signal. in spite ofthis, the level of the defect signal 49 is raised above the voltagelevel at which the pulse generator 39 is triggered by an integratornetwork or stage 41. The simplest form of this network consists of acapacitor Ci and resistor R1. When the time constant Ci-Ri is of theorder of the duration Ti of the defect signal, the triggering voltagelevel is safely reached within the time interval Ti as shown by thesignal 49.

The statistical average of the noise signal 44 consists of equalpositive and negative voltage excursions so that its integrated meanlevel 48 is constant and is below the triggering voltage level. When theextent of the taint defect in the direction of the paper travel varies,the required integrating time constant Ti is changed in proportion.

Advantageously, an amplifying integrating stage is used as shown in FIG.5. The output stage 50 of the amplifier 37, as shown in FIG. 4, isconnected by coupling capacitor 51 and resistor 52 to the control gridof the integratoramplifier pentode 65*. The cathode bias resistor 55,gridleak resistor 54, screen feed resistor 57 and plate load resistor 59all have such values that the gain of this pentode stage is severalhundred when the feed-back resistor 53 is disconnected. By connectingthe plate to the grid by means of the coupling capacitor 56 and feedbackresistor 53, the gain of the pentode stage is set at approximately 20.At the same time, the linear amplification of any one frequencycomponent is improved. The integrating time constant is defined by theplate load resistor 59 and capacitor 61. The integrator capacitor 61 hassuch value that it builds up the unidirectional defect signal amplitude.6 while the noise signal consisting of peaks of alternating polaritiesand having a higher frequency content, is not integrated and isattenuated. The inputs of several such integrating amplifiers withvarious time constants may be connected to the plate of tube 59 and theoutputs connected by adder resistors and an adder amplifier to thecontrol grid of the pulse generator.

The output pulse produced by a defect located at position D, see FIG. 2,is delayed until the leading edge of the continuous sheet material 2 hasapproached position F where the deflector edge 17 of grading switch 16is located. Meanwhile, the sheet 2 has been cut by blades 13 and 14 atthe position indicated by the letter E. The aforementioned delayconsists of a constant part and a variable part. The constant part ofthe delay is defined by the time required for the defect in sheet 2 tomove from the position D to the position E. For example, when thevelocity of travel of the paper is 200 inches per second and thedistance from D to E is twenty-five inches, such constant part of thedelay is 25/200 which equals 0.125 part of a second. The variable partof the delay is defined by 3 parameters, namely the angular position ofthe rotating cutter member 14 at the instant the defect arrives atposition E, the distance from E to F and the speed at which the cutsheet is carried by the tape 15.

In what follows, it will be seen that a thyratron 64, see FIG. 6,associated with microswitches 62, 74 and 75 together with relays 68 and7 and grading switch actuating solenoid 72 will satisfy the conditionsset by the said parameters for reliable sorting.

Initially, mioroswitches 74 and 75 are closed, microswitch 62. is open,thyratron 64 is biased to cut-oif by having its control grid connectedby resistor 66 to adjustable potential divider 67 which is connectedbetween ground and a negative supply rail while coils 68, 7G and 72 arenot energized and contacts 69, 71 and 73 are open. Thyratron 64 is firedby a positive pulse delayed relative to the instant of the leading edgeof pulse 43 by the constant pant of the delay. See FIG. 4.

By using a phantastron delay generator which is well 10 known in theart, pulse 43 starts the plate voltage rundown of a normally cut-oifphantastron which bottoms at the end of the constant delay and thenreturns to its initial cut-off condition. The diiferentiated screenvoltage jumpat this instant is the positive pulse 76 which fires thyratron 64. See FIG. 6.

The variable second part of the delay is introduced by synchronizing theinstant of operation of the solenoid of the grading switch 16 with therotating cutting blade 14. When the cutting blades 13 and 14 meet,microswitch 62 closes and the increased current through relay coil 68closes contacts 69 which energizes relay coil 70 which, by closingcontacts 71, connects solenoid 72 which actuates grading switch 16.Contacts 73 close simultaneously with contacts 71 holding coil 70energized until microswitch 74 momentarily opens. The momentary openingof microswitch 74 is synchronized with the rotating cutter blade, thus,for example by placing microswitch 74 diagonally opposite microswitch62, it initiates the cycle of operation. By this means then, the gradingswitch 16 is retained in the desired position for the passage of half alength of the cut sheet in the desired position. The edge 17 of thegrading switch 16 is so shaped and its movement is such that it does notinterfere with the passage of the cut sheets once they are deflectedtoward either the second grade compartment 19 or the first gradecompartment 26. Further, thyratron 64 is prepared for the reception ofsome other defect pulse by the momentary opening of microswitch 75 whichinterrupts the HT supply to thyratron 64. The angular position of switch75 is displaced a few degrees clockwise relative to the position ofswitch 62. while switchm 62, 7d and 75 are operated by one or moreshaped disc members attached to the mechanism of the rotating cutter 14.The rotation of the cutting blade 14 is anticlockwise. In FIG. 7, threeshaped disc members are shown together with microswitches 62, 74 and 75,however, other switches such as the commutator type with brush pick-upsmay be employed.

When the distance between the cutting position E and point F of thegrading switch is greater but is of the order of the length of the cutsheets, the hereinbefore described method and means for producing therequired delay is satisfactory. Other and more complex means may beemployed when the speed of the carrying tapes 15 is not constant or whenthe distance between points E and F is much greater than the length of acut sheet. In all cases, the delay must be such that the grading switchis deflected just before the leading edge of a defective sheetapproaches the edge 17 of the grading switch 16.

In the operation of the invention thus far described, sheet paper 2 isfed from a large roll to auxiliary rollers 3 and 4, such rollerseliminating flutters and folds in the paper sheet. The paper then passesover inspection member 5 after which, by means of draw rollers 11 and12, the paper sheet is moved from the inspection member 5 to the point Ewhere it is cut into sections or lengths by means of cutting members 13and 14. Immediately above inspection member 5, the paper 2 is floodedwith strong, uniform light by means of the fluorescent lamps 8. Thelamps 8 are arranged close to the paper surface and on opposite sides ofthe long, narrow aperture 7 so that the light rays therefrom are.directed downwardly on to the surface of the paper on the inspectionmember 5 and are then reflected upwardly through aperture 7 to thephotoelectric cells 6.

As the sheet paper moves over the inspection member 5 and a defectappears in that portion of the paper beneath the aperture 7, thediffused, reflected light rays falling upon the photoelectric cells 6will increase or decrease the current of the photoelectric cellsaccording to the nature or characteristics of the defect and suchchanges in the current produce a corresponding voltage change such assignal 36 at the input of amplifier 3-7. The amplified signal 38produced by the direct channel, or

.also shown in FIGS. 2. and 3.

11 signal 42 produced by the integrator 41, passes the generator 39 andproduces output pulse 43 which, in turn, actuates grading switch 16.

At the instant the defect in the paper appears beneath aperture 7,output pulse 43 is produced. This output pulse is then delayed by a timeperiod which is defined by the time period in which the said defectmoves from beneath aperture 7 to the stationary cutting blade 13. Thegrading switch 16 moves upwardly at the instant the stationary androtary cutting blades 13 and 14 meet to cut the defective sheet ofpaper. The defective sheet of cut paper is then carried to compartment19 by tapes 15. When the cut sheets of paper are without defect, thedeflector 16 remains stationary and such first grade paper is carried tothe first grade compartment 20 by tapes 18.

Referring now to FIG. 8 of the drawings, the lines numbered 77, 78, 79,80, 81, 82, 83 and 84 represent the photoelectric cells shown in FIG. 2and FIG. 3, such lines also give visual representation of the aperture 7In practice, lines 77 through 84 meet and represent a part of thecontinuous aperture 7. The lines 77 through 84 also indicate equalsections of the sheet material under inspection, each such section beingviewed by one, two or more photoelectric cells. The number ofphotoelectric cells viewing each section from 77 to 84 are equal.

The cathode of the photoelectric cells, as for example seen at numerals23 to 26 in FIG. 4, viewing sections indicated by numerals 77 and 79,are connected together and are connected by link 85, which may be acathode or anode follower stage, such as is indicated by numeral 32shown in FIG. 4, to one of the input control grids of amplifier 87 whichconsists of the heretofore described long-tailed pair stages.

Similarly, photoelectric cells viewing sections 81 and 83 are connectedtogether and are connected by link 86 to the other input control grid ofamplifier 87.

Sections 78, 80, 82 and 84 are interleaved with sections 77, 79, 81 and83 and the photoelectric cells viewing sections 78, 80 and '82, 84 areconnected by links 88, 89 to the two input control grids of amplifier90. Amplifier 90 is identical in all its characteristics to amplifier87.

According to my invention, the main rule of connections is that adjacentsections such as 77, 78 or 79, 80 and so on, are connected to differentamplifiers and that to each control grid of such amplifiers the samenumber of photoelectric cells are connected and thus view sections ofequal lengths. The number of amplifiers used can be two or more,however, the number of sections used must be an even number. As anexample of this, an alternative arrangement satisfying theaforementioned principle of connection is shown in FIG. 9.

When the number of the equal interleaved aperture sections is n, themaximum number of amplifiers which can be connected according to thestated rule is /zn, and the minimum number of amplifiers is 2 when n/ 2interleaved sections are connected to one amplifier and the other halfto the second amplifier; n must be an even ninnber in order to preservethe symmetry of the system and should preferably be equal to 4m, Where mcan be any number. As an example, when m equals 1, 2, 3, 4, or 5 nequals 4, 8, 12, 16 or 20.

Adjacent aperture sections are connected to difierent amplifiers inorder to eliminate blind spots in the viewing aperture 7 shown in FIGS.2 and 3. To serve the same purpose, the photoelectric cells associatedwith one section also view bordering parts of bothadjacent sec tions. Ifadjacent sections, as for example It and section n1, were connected tothe two control grids of the same amplifier, and if a defect then passesunder the dividing line of the said sections, the amplifier would receive two identical inphase signals and such signals would not beamplified.

An' amplifying system such as block '87 or 90 seen shown in the circuitdiagram of FIG. 10. This amplifier system performs all the basicfunctions which are specified in the amplifying system shown in FIG. 4and is immune to in-phase interference signals, amplifies and detectsdefect-signals brighter or darker than the mean intensity of the sheetmaterial under inspection. It is not sensitive to supply and heatervoltage variations. While it is also an AC. amplifier it issubstantially free from blocking effects, i.e., it reliably detectssmall defects which follow large and intense defects, its integratorchannels do not respond to the basic paper noise and are sensitive tounidirectional weak defect signals which extend in the di rection oftravel of the sheet material. Additionally, the output cathode followerstages of several such amplifying systems are paralleled and connectedto the output pulse producing stage common to all amplifying systemssuch as block 39 seen in FIG. 4, in such a manner that undesirableinteraction between the amplifying systems, does not occur.

Photoelectric cells .98 and 99 represent the cells which view oneaperture section, e.g. section n2 of FIG. 9. Photoelectric cells 100 and101 view another aperture section, eg 11 seen in FIG. 9. For purposes ofdescription only, two pairs of photoelectric cells shown in FIG. 10 viewequal interleaved aperture sections. The number of photoelectric cellswhich view each such aperture may be one or more.

The cathodes 102 and 103 of cells 98 and 99 are connected to loadresistor 106 as well as the control grid 108 of the cathode follower110. The photocathodes 104 and 105 of photoelectric cells 100 and 101are connected to the load resistor 107 as well as control grid 109 of asecond cathode follower 1 11. The resistance of resistor 106 is equal tothat of resistor 107.

' The DC. voltages developed across the load resistors 112 and 113 ofthe cathode followers and 111 are proportional to the diffused reflectedlight from the sheet material viewed by photoelectric cells 98, 99 and100, 101. The output of the photoelectric cells 98, 99 and 100, 101 arematched to 'be equal by adjusting the variable arms 1'14 and 115 ofcathode follower load resistors 112 and 113. Thus, the in-phase outputsignals of the inspection head are matched and are applied "to bothinput control grids 118 and 119 of the amplifier.

The amplifier proper consists of long-tailed coupled triode stages,three such stages being shown in FIG. 10. 'Triodes 120 and 121 representthe first long-tailed pair, triodes 122 and 123 the second pair andtriodes 124 and 125 the output stage. The number of stages used willvary according to the gain and stability requirements of the amplifier.The plates of the triodes are connected to the positive supply line +-HTby resistors 126, .127, 128, 129, and 131. Such load resistors belongingto the first, second and output stage have equal resistances, e. g. theresistance of member 126 equals the resistance of member 127.

The cathodes of the paired triode stages are connected by resistors 132,133 and 134 to each other and are connected by the long-tail resistors135 to to the negative supply line -HT. The cathode resistors have equalresistance values for each paired staged. Thus the resistance ofresistor 135 is equal to the resistance of element 136, the resistanceofelement 137 is equal to the resistance of element 138 and theresistance of element 139 equals the resistance of element 140.

The gain of the first stage is mainly defined by the resistance ratio ofresistors 126/132 or 127/ 132 giving an identical number, the secondstage gain by 128/133. The third stage gain is by 130/134. Therefore,the gain of the whole amplifier is defined by the value of theseresistances and is substantially independent of the triodecharacteristics as well as aging loss of electron emission from thecathodes and heater voltage changes in these tubes. It is an essentialfeature of my invention that when a particular defect is sensed by oneor more photoelectric cells 93 to 1 and is amplified to some welldefined level by the constant gain amplifying system shown in FIG. 10,all other amplifying systems associated with the rest of thephotoelectric cell row 6, shown in FIGS. 2 and 3, must have the samegain and characteristics as the amplifying system shown in FIG. 10,raising the defeet signal to the same well defined lev'el. According tothe present invention and in order to make all the amplifier systemsidentical with each other, precision resistors are employed and suchresistors define the constant gain of the amplifiers. All amplifierstherefore are made to provide the same gain so that the differences intube characteristics will have only a second order effect upon suchgains and the gain frequency characteristics of all amplifiers will beidentical. Discrepancies of the nominally identical precision resistorsare compensated for by a slight adjustment of the variable resistor 132which equalizes the aht of all the amplifiers.

The amplifying stages herein are direct-coupled to each other byresistors 141 through 144. The resistance of element 141 equals that ofelement 142 and the resistance of element 143 equals that of element144.

The control grids of the amplifying stages are connected to the negativesupply line HT by resistors 145 through 150. The resistance of element145 equals that of element 146 andthe resistance of element 147 equalsthat of element 143 while the resistance of element 149 equals that ofelement 159. The Values of resistors 141 through are so selected thatthe voltage levels of the control grids 118 and 119 and the controlgrids of tubes 122 through 125 are close to ground potential.

By equalizing the values of all the resistors attached to triodes 111,121, 123 and 125 with the values of the corresponding resistors oftriodes 110, 126, 122 and 124, the amplifier is made insensitive tovariations of the sup ply potentials +111 and HT. Supply voltagevariations are reduced to in-phase signals at the control grids, thecathodes and the plates of the triode-pairs, hence such signals are notamplified.

The frequency response of the amplifying system should be constant from1 cycle per second to 9 kilocycles. The low frequency limiting responseof 1 cycle per second is defined by the requirement of the system to beable to integrate information on the sheet material when it travelsslowly. When the slowest velocity of the sheet material is inches persecond, information present in the section of sheet material 20 inchesin length can be integrated. The limit of high frequency response isdefined by the requirement of amplifying without attenuation of thesmallest defect of lip inch by X inch at the fastest sheet velocity.When the velocity of the sheet is 200 inches per second the highfrequency limit must be 3 kilocycles per second and at a sheet velocityof 601') inches per second the high frequency must be 9 kilocycles persecond.

The amplifier must also respond without blockage to small signals whichfollow high amplitude signals, for example, when 21 & inches diameterfull-black spot follows a defect of a much larger area.

In order to satisfy the low frequency requirements, the arnplifier shownin FIG. 10 is DC. coupled from the photoelectric cell cathodes to theoutput plates of the tried-e pair indicated by numerals 124- and 125. Itis a further essential requirement of the output triode pair 124 and 125that the steady state potentials of such output plates 152 and 153 shallbe equal. These potentials are made exactly equal by balancing potentialdivider 154. The plate potentials must, of course, be equal in orderthat the coupling diode network, which consists of diodes 155 through169, will function satisfactorily.

As is well known in the art in DC. coupled high gain amplifiers, it isdifficult to achieve the stated balance of the output potentials forlong periods of time without rebalancing such systems. Therefore,according to my invention, the DC. amplifier shown in FIG. 10 isconverted into an AC. amplifier by a feed-back network consisting ofresistors 165, 166, 167, 168 and capacitor 169. Substantially all theDC. output at plates 152 and 153 is fed back to the input grids 118 and119 by this network while the useful signal frequencies from one cycleper second to 9 kilocycles per second are decoupled by capacitor 169 andconsequently are not fed back from the output to the input. As a resultthe system amplifies without the feed-back attenuation of suchfrequencies.

Blockage of the amplifier previously mentioned may be avoided by Zenerdiodes 155 and 156 connected to the output plates 152 and 153, or,alternatively, a neon tube could replace diodes 155 and 156. Further,the

gain of the amplifier is such that the smallest defect signal isamplified to say a 50 volt level. The voltage breakdown of the Zenerdiodes or the neon tube is so chosen that any output signal above the 50volt level can not raise the potential of plates 152 and 153 due to thedischarge in such diodes or neons. As a result the A.C. negativefeed-back pass does not receive signa s above the minimum defect leveland can not induce blockage.

Additionally, the steady state DI). levels of the output plates 152'.and 153 are substantially defined by the precision resistors 165, 167,145 and 166, 168 and 146, when the supply voltages +HT and HT areregulated and have the long-tern stability of plus or minus one volt,the output plate potentials are defined with an accuracy ofapproximately plus or minus a volt and are equal, assuming, of course,that the resistance of resistor equals that of element I456 and 167equals that of element 168 while element 145 equals element 145.

When a black defect in the sheet material reduces the light collected bythe photoelectric cells 98 or 99, a positive sign-a1 indicated bynumeral 17% is produced at the output plate 1:32. Simultaneously, thecathode coupled pairs shown in H6. 10 produce an identical negativeoutput signal indicated by numeral 169 at the output plate 153, suchsignal is negative only relative to the plate mean potential. It will beseen therefore that signals due to any defect result in two outputsignals which are equal and are not in-phase. When a defect viewed byany of the photoelectric cells 98 through 39 1 is brighter or darkerthan the mean intensity of the sheet material, one of the output plates152 or 1531's always positive while the other is negative. Signalsindicated by the numerals 171 and 172 represent such push-pullanti-phase signals; the negative section of the output signals 169 andthe positive section 176 are due to the defect and sections 173 and 174are due to the basic noise of the sheet material.

Each of the output signals just mentioned are fed by two channels to theoutput pulse generator which may be a S-chrnitt trigger generator whichis well known in the art and which consists of double t-riodes 175 and176.

Channel one of the invention is the amplitude differentiating channeland its operation is described in what follows. The potential of thecathodes of the triodes 175 and 176 is defined by potential dividerchain 177, 173 and 179. Adjust-able resistor 179 sets this potential tosay 250 volts; triodo 175 is normally cut off so that the conductingtriode 176 acts as a cathode follower. Potential divider 189 defines thepotential of the cathode of the output cathode follower 131 of theamplifying system and is, for example, set at 225 volts. The 25 voltdifference between the cathode potential of triodes 175 and 176 and thecathode potential of cathode follower 131 is sufficient to cut offtriode 175.

The potential of the control grid of triode 181 is defined by thepotential divider 189* through the grid-leak resistor 182 and is closeto the potential of the cathode of triode 131 for the reason that triode131 is a cathode follower. For purposes of description, the gridpotential is assumed to be 223 volts. This potential of 223 volts isalso applied through resistor 183 to the junction of the 15 output platecoupling diodes 157 and 158. The anodes of the diodes 157 and 158 areconnected to the output plates 152 and l53 while the cathodes areconnected to the grid of the output cathode follower 181 throughresistor 183. Therefore the cathodes of the said diodes are biased 23volts positively relative to the steady state output plate potentialswhich are set at 200 volts by the previously described feed-backnetwork. The peak values of the defect signals 169 and 170 are limitedto plus or minus 50 volts relative to the mean output plate potentials.by the Zener diodes 155 and 156. Therefore, the peak level of theoutput signal 169 is +150 volts and that of the output signal 170 is+250 volts. Peak values of the basic noise signals 173 and 174 are lessthan plus or minus volts relative to the mean output plate potential sothat the noise signals do not pass through the biased coupling diodes157 and 153 while a portion of the defect signal 170 will pass throughcoupling diode S and will appear at the cathode of the cathode follower181 and at the grid of the trigger generator triode 175. Such a defectsignal is approximately 50 volts minus 23' volts which equals 27 voltsand its peak level at the grid of triode 181 is +150 volts and at thecathode of triode 181 it is +252 volts. Thus the control grid of thenormally cut-off triode 175 of the trigger generator is raised above thepotential level of 250 volts of its cathode and the output pulse 184 isproduced. The relative potentials of the cathodes of the triggergenerator triodes and the cathodes of the coupling diodes are thus soarranged that defect signals raising the potential of one of the outputplates to about +250 volts will produce an output pulse of constantamplitude (184), while the noise content of the output signals do notelfect the output trigger generator.

The second channel of the amplifying system shown in FIG. 10 consists ofcoupling diodes 159 and 168, paralleled by resistors 161 and 162 withcapacitor 163 coupling the junction of the anodes of diodes 159 and 160to the control grid of the Miller integrator 185 through resistor 186.The following components associated with the Miller integrator are thegrid-leak resistor 187, plate load resistor 188, the plate to grid teedback capacitors 189 and 190 which are coupled to the control grid byswitch 191, the integrator output coupling diode 164, the plateresetting diode 192 and the screen grid de-coupling capacitor 193.

The principle of the Miller integrator is well known in the art and itsmain function as illustrated in FIG. 10 is the production of a linearlyincreasing plate potential. The rate of increase of the plate potentialFLY-L dT C' R Where v is the potential applied to the resistor 186 atthe junction of resistors 186 and 187 and capacitor 163, C is thecapacitance of the capacitors 189 0r 19! and R is the resistance ofresistor 186. The control grid and the cathode of the Miller pentode 185are at substantially the same potentials while the resulting platecurrent is such that the plate potential of tube 185 will be only a fewvolts, say ten volts, above ground level (cathode potential). Byapplying to the screen grid of this Miller integrator a high enoughpotential by divider 194, such bott-omed condition of the plate canalways be attained.

Now when a negative potential is applied to R (186), the current of thetube is reduced and its plate potential will linearly increase towardthe positive line potential +HT set at say +350 volts. When the platepotential reaches l+223 volts, that is the level to which the grid ofthe cathode follower output tube 181 is set, coupling diode 164 conductsand the further rising plate potential of the integrator 185 appears atthe output of the amplifying system producing an output pulse similar tosignal 184, the duration of which is equal to the time period duringwhich the integrator plate potential remains above 250 volts.

This second integrator channel of the amplifying system senses defectsignals which are below the trigger level of the output pulse generatorand which can be of the same order of magnitude or smaller than thebasic noise level (173, 174) of the sheet material under inspection andwhich are due to defects which are extended in the direction of travelof the sheet material. For example, the integrator constant CR may beset to have such value that the input signal v specified in the tablebelow raise the integrator plate potential to the hereinbefore definedtrigger level of 250 volts for defects specified in the following table.

Dimension of the defects at right angles to the direction of travel ofthe sheet material equal of an inch.

Time intefgval Dimension of in millisecond defect in the during whichIntensity of defect, direction of Input signal plate of intepercent fulltravel of the amplitude 11 grater rises to black in percentage sheetmaterial in volts +250 volts,

in inches sheet velocity equals 100 inches per second A0 25 0.625 ls 12.5 1. 25 ,4 6. 25 2. 5 V: 3.125 5. 0 1 1. 56 10. 0

For defects extending in the direction of travel of the sheet material,the integrator response is the same as that of the amplitudedifferentiating channel, channel 1, for defects extended at right anglesto the direction of travel of the sheet material. In both channels theoutput signal peak is proportional to the product of the intensity andthe area of the defects. This law corresponds quite Well to the mentaleffect whereby the human eye senses the magnitude of a defect.

The dimensioning of the time constant CR of the integrator in the abovedescribed manner is given only as an example. The time constant can bechosen to satisfy other integration requirements. For example, tointegrate a defined number of minute defects (each less than inchdiameter) falling Within a certain area of the sheet material. One ormore integrator stages satisfying several programmes can operatesimultaneously and can be coupled via capacitors such as 163 and diodessuch as 164 to the output cathode fol-lower 18 1.

When [the defect signal is smaller than the amplitude of the noisesignals such as are indicated at 173 and 174, it is desirable that theintegrator shall not respond to the noise signals. According to myinvention this is accomplished by coupling the output plates 152 and 153by diodes 159 and 160 as well as capacitor 163 to the input of theintegrator. The cathodes of the diodes 159 and 160 are connected to theoutput plates while the anodes are connected to the capacitor 163. Themean potential of the output plates is fed to the anodes of the diodes159 and 1-60by two equal resistors 161 and 162 so that the couplingdiodes are not biased and only the negative excursions of the noisesignals 173 and 174 pass through the coupling diodes and appear at thejunction of the capacitor 163 and diodes 159 and 160. The resultingsignal is the rectified noise signal and thus its peak to peakamplitudes are halved. The time constant of the capacitor 163 and theresistors 186 and 187 is chosen in such a manner that'it satisfies thelow frequency response requirement of the amplifying system and as anexample it is made three seconds. Therefore, the rectified continuousnoise signal charges in three seconds the capacitor 163, thus biasingthe coupling diodes 159 and 160 by a voltage level which is of the orderof the rectified continuous noise amplitudes so that such noise signalsare substantially eliminated at the input of the integrator as shown bysignal 197. The integrator input signal 196 does not have the saideffect upon diodes 159 and 160 and therefore is not attenuated.

According to the present invention, the screen potential of theintegrator 1 85 is so selected that its current is cut oil when only afew volts are applied to the control grid of the integrator. Varioussheet materials have diiferent noise contents. Some sheet materials havea rather nonuniform noise content in which case the sensitivity of theintegrator has to be reduced thus limiting the sensitivity of the systemfor the detection of the defects extending in the direction of travel ofthe sheet material. Awording to the present invention, the potentialdivider 194 is adjusted to various levels to match the noise level ofthe various materials. When the screen potential of the integrator israised, the grid cut-off potential is increased. Therefore, according tomy invention the noise sensitivity contro member 194 is adjustedaccording to the noise characteristics of the various sheet materials.

As an example of this, when the peaks of noise signals 197 are 2 volts,the noise sensitivity control 194 is set in such a manner that thecontrol grid cutoff potential of the integrator is say minus 5 volts,then the plate potential of the integrator will not reach the +223 voltsat which level the coupling gate, diode 164, opens.

In accordance with the invention, the control grid, and hence the plateof the integrator 185 is switched to ground potential periodically bypositive pulses 198 or 2% which are applied through coupling diode 192.Pulse 209 is initiated by the delayed output pulse 184.

When, for example, the conditions of integration programmed are suchthat defect-information contained in each one-inch length of the sheetmaterial (having a width defined by the sections viewed by thephotoelectric cells 98 and 99) is to be integrated, it is possible thata one-inch length containing defects which raise the plate potential ofthe integrator above the trigger level of the output pulse generator isfollowed by a one-inch length of the sheet which, while containing minordefects, should not be discarded as defective. The characteristics ofthe integrator circuit are such that such sections containing minordefects and which are not to be discarded could also be sensed asdefective sections, unless the output pulse 184 or sampling pulses 193return the integrator to its steady state, namely, when the potential ofits plate is close to ground.

This effect is better understood by reference to FIG. 11 and by assumingin the example which follows that specific voltage levels are applied tothe integrator. Let the sheet velocity be 100 inches per second with afaint defect one-inch long in the direction of the travel of the sheetmaterial. Then let the signal v corresponding to this defect be volts atthe integrator input and let the control-grid cut-ofi potential of theintegrator be -2 volts. The time constant of the integrator is soselected that when v=10 volts, the integrator output reaches thepositive supply potential +350 volt-s in 10 milliseconds;

350 volts l0 volts 10- sec. CR

Therefore, the trigger level of 250 volts is reached in 6.7 millisecondsand the resulting duration of the output pulse 134 would normally be 3.3milliseconds while the defect passes under the aperture 7. Let thesteady state bias of the control grid, due to grid current, be 1 volt;after 10 milliseconds the control grid'reaches 2 volts which is thecut-off potential of the integrator. At the instant the defect passes,beyond the aperture (1 in FIG. 11), the output potential starts to fallslowly; the voltage drop corresponds to v=+2 volts, because when thedefect signal drops to zero the potential across resistor 186 is +2volts while during the presence of the defect signal it was 9 volts.Thus for a period of approximately lSrn-illiseconds the integratoroutput voltage remains above the +250 volt trigger level and only aftera considerably longer time period can the initial potential (+10 volts)of the integrator plate be reached.

Conditions are made worse when during the slow recovery of theintegrator plate minor defect or noise signals, say of the order of 2volts occur, such signals should not be detected. Such signals howeverwill retard the recovery of the integrator as seen in FIG. 11. Due tosuch slow recovery, the trigger generator is kept in its trigger .statelonger than required and as a result minor deflects can causeretriggering.

The slow recovery of the integrator after the termination of a defect isa basic and essential feature of an integrator but by its very nature itis also responsible for the above described sluggishness of the triggergenerator. By applying the positive pulses 198 or 200 to the controlgrid of the integrator such sluggishness is eliminated. In the examplegiven the PRF, or pulse repetition frequency, f of sampling signal 198is 100, then each one-inch length of the sheet material isintegrated.The PRF of the sampling pulses f is adjusted to be proportional to thesheet velocity and to be inversely proportional to the sheet lengths tobe integrated; that where W is the sheet velocity and L is the length ofthe sampled sections in the direction of travel of the sheet material.

Alternatively, when defect signals raise the integrator output to thetrigger level, the resulting trigger pulse is delayed and is used afterconditioning in sampling pulse generator 199 as the resetting pulse atthe control grid of the integrator. Such time relay (Td defines theWidth of the output pulse 184. The delay Td is so selected that thewidth of the output pulse 184 is of the shortest duration compatiblewith the satisfactory operation of the control circuitry of the gradingswitch 16 shown in FIG. 6, or is compatible with other applications ofthe output pulse 184. The integrator resetting pulse or pulses,according to either alternative, are produced in sampling pulsegenerator 199 which is shown in block form only in FIG. 10. Thisgenerator can, for example, be a transitron Miller pulse generatorassociated with a phase inverter both of which are well known in theart. Such generator also produces either the in-time equally spacedsampling pulses according to the first alternative or according to thesecond alternative is triggered by the leading edge of pulse 184,completes its Miller run-down defining delay Td and at the end of suchrun-down generates the resetting pulse. 1

The width of the resetting pulse 200 or sampling pulses 198 can bevaried by a multi-position switch in pulse generator 199 by convenientlyswitching a number of capacitors coupling the suppressor and screengrids of the transitron Miller pentode. The width-s of the resetting orsampling pulses are defined by the velocity of the sheet material underinspection and by the charging time of capacitors 189 or 190, the saidpulse Width must be long enough to allow the return of the plate currentdischarging capacitor 189 or 190 rapidly. Thus,

the resetting period is only a small fraction of the sampling orintegrating period.

In accordance with the presentinvention, the cathode ing capacitors 139or 190.

of silicon junction diode 192 is connected to the control grid of theintegrator while its anode is connected to the cathode of the cathodefollower 201. As previously stated, the potential of the control grid ofthe integrator is at approximately 1 volt. With the aid of the potentialdivider resistors 202 and 203, the control grid of the'cathode follower201 is biased through resistors 204 to say 20 volts, so that the cathodeof unit 201 will be approximately volts. Thus the coupling diode 192 isbiased in such a manner that when the negative input potentials v areapplied to resistor 186, diode 192 is not conducting and thereforerepresents a perfect gate having a resistance of 1000 megohms or more.The perfection of diode 192 in this non-conducting phase is essentialfor the reason that the resistance of resistor 186 is several megohmsand the leakage path of the control grid must be of greater magnitude inorder to not eifect the integrator. By employing the silicon junctiondiode 192, this condition is well satisfied.

The resetting positive pulses 205 and 206 raise the control grid of unit201 above ground level and hence also the potential of the cathode ofunit 201. Thus, the resetting pulses 198 and 200 pass through the nowconducting diode 192 and drive the control :grid of the integrator'185into the grid-current region quickly discharg- In FIG. 10, only twointegrator capacitors 189 and 190 are shown. In order to cover therequirements of integration at sheet velocities of from inches persecond to 600 inches per second and integrating l to 20 inch lengths ofthe sheet material, a

large number of integrating capacitors are served by switch member 191.

Further, according to my invention, the diode coupling networkconsisting of diodes 157, 158, 159, 160 and 164 represent near idealgates for the various functions which the system has to perform. Forexample, in the presence of the positive output signal 170 diode 158conducts and diode gates 157 and 160 are closed thus separating the opengate 159 from the signal source which opened diode 158. The negativepulse 169 then passes through the conducting diode 159 to the input ofthe integrator and the positive output signal 170 driving the outputcathode follower 181 does not interact in any way whatsoever with thenegative output signal 169 which is fed to the integrator. Should thenegative output signal 169 raise the integrator gate above the level ofthe peak value of the output signal 170, the gate 164 opens and theresulting output pulse 184 is beneficially reinforced. The two channelsof the amplifying system support but never interfere with each other.When the phase of the output signals is opposite to those of signals 169and 170,-gates which were open in the previous example close and thosewhich were closed will open while the function of gate 164 remainsidentical.

When the output signals, due to faint defects, are below the triggerlevel of the output pulse generator and are detected by the integratorchannel and as soon as the rising plate potential of the integratorreaches the biasing level of coupling diodes 157 and 158, the latterdiodegates' close and the integrator output does not effect the signalcontent at the plates 152 and 153.

The outputs of both channels are fed via resistor 183 to thegcontrolgrid of the cathode follower 181. The purpose of this resistor is tolimit the consumption of the cathode follower 181 when the integratoroutput approaches the positive line potential +HT.

The use of coupling capacitor 163 is not essential. The outputs of gates159 and 160 can be coupled direct to the integrator by a potentialdivider which is con nected between the junction of diodes 159 and 160and the negative line 'HT. The point of such potenti-al divider, whichis at ground level, could then be connected to the junction :ofresistors 186 and 187. Other direct coupled arrangements are possible bymodifying the circuit'of the integrator shown in FIG. 10. When such 178,179,207 and 209, capacitors 210 and 211, the integrator sampling andresetting generator 199, resetting pulse cathode follower 201 associatedwith resistors 202, V

203, 204 and 212, potential divider 194control1ing the noise sensitivityof the integrator and the potential divider 180 biasing the couplingdiodes 157 and 158 of channel one, represent a separate control unitcommon to all amplifying systems each of which is identical to thatshown in FIG. 10. The outputs and inputs of the said control unit areied to many amplifiers by terminals 213, 214, 215, 216, 217, 218 and219. The corresponding connections of many amplifying systems can alsobe connected to the said terminals, and such amplifying systems do notinteract with each other. With the present invention, whicheveramplifier system first detects the defect, and which in turn raises theoutput potential of its cathode-follower 181 above the mean level of say225 volts, will switch off all other output cathode-followers paralleledat terminal 218.

Changes may be made in the above and many apparently widely differentembodiments constructed without departing from the spirit or theessential characteristics of the invention.

What I claim is:

51. In apparatus for detect-ing defects in moving sheet materials havingsubstantially uniform reflection characteristics, which senses, byphotoelectric means, variations in the intensity of light reflected fromthe sunface of such a material as it'passes over an inspection surface,an improved inspection head comprising an aperture plate mountedparallel to said inspection surface, and closely adjacent thereto, meansdefining an elongated narrow aperture in said plate extendingtransversely to the direction of motion of said material over saidinspection as the diameter of the smallest and most intense defect.

Y to be detected, and said aperture having a dimension transversely tothe direction of motion of the sheet material substantially greater thanthe size of the average texture discontinuity of the unblemished sheetmaterial, and of the order of magnitude of the length of the faintestand narrowest defect extending at right angles to said direction ofmotion which produces a variation upon the intensity of the lightreflected from said surface of said material substantially equivalent tothat produced by said smallest and 'most intense defect to be detected.7

2. In apparatus for detecting defects in moving sheetmaterials havingsubstantially'unifonm reflection characteristics, which senses, byphotoelectric means, variations in the intensity of light reflected fromthe surface of such a material as it passes over an inspection surface,an improved inspection head comprising a light-tight housing mountedabove said inspection surface and having a lower surface parallelthereto, an elongated narrow aperture in said lower surface extendingtransversely to the direction of motion of said material over saidinspection surface, the dimension of said aperture in the direction ofmotion of the sheet material beiug of the same order of magnitudeversely to the direction of motion of the sheet material substantiallygreater than the size of the average texture discontinuity of theunblemished sheet material, and of the order of magnitude of the lengthof the faintest and narrowest defect extending at right angles to saiddirection of motion which produces a variation upon the intensity of thelight reflected from said surface of said material substantiallyequivalent to that produced by said smallest and most intense defect tobe detected, substantially identical first and second illumination meanspositioned respectively on each side of said aperture and equidistanttherefrom throughout the length thereof, a plurality of photoelectriccells mounted in series inside said housing above said aperture andresponsive only to diffused light entering said housing through saidaperture, and shielding means integrated with said housing extendingdownwardly therefrom and terminating closely adjacent to the surface ofsaid material passing over said inspection surface whereby to restrictillumination of said material surface passing beneath said aperturesubstantially to that provided by said illumination means.

3. In apparatus for detecting defects in moving sheet materials whichsenses, by photoelectric means, variations in the intensity of lightreflected from the surface of such a material as it passes over aninspection surface, an inspection head having a lower surface positionedabove said inspection surface and parallel thereto, means defining anarrow elongated aperture in said lower surface extending transverselyto the direction of motion of said material over said inspectionsurface, illumination means for illuminating that section of saidmaterial passing over said inspection surface beneath said aperture, aplurality of photoelectric cells mounted above said aperture andresponsive only to light reflected from said section of material passingthrough said aperture, said inspection surface having a convex curvaturein the direction of motion of said material, said illumination meansbeing positioned in or below that plane passing through said apertureparallel to the plane tangential to said curved inspection surfaceimmediately adjacent said aperture.

4. In apparatus for detecting defects in moving sheet materials havingsubstantially uniform reflection characteristics which senses, byphotoelectric means, variations in the intensity of light reflected fromthe surface of such material as it passes over an inspection surface, aninspection head having a lower surface parallel and closely adjacent tosaid inspection surface, means defining an elon gated narrow aperture insaid lower surface extending transversely to the direction of motion ofsaid material over said inspection surface, electrically energizedillumination means illuminating that section of said material passingover said inspection surface beneath said aperture, photoelectric meanspositioned adjacent said aperture and responsive only to light from saidillumination means reflected from said section of material passingthrough said aperture, an A.C. amplifier responsive only to inputsignals lying within a given frequency range and having its inputconnected to said photoelectric means, and a source of electrical energyfor said illumination means, the frequency associated with any variationof light output from said illumination means eifectively lying outsidethe said frequency range of said amplifier.

5. Apparatus as defined in claim 4 wherein said source of electricalenergy is a regulated DC. power supply. 7

6. Apparatus as defined in claim 4 wherein said illumination meanscomprises tubular fluorescent lamps having their longitudinal axesparallel to said inspection surface and said source of electric energyis a regulated DC. power supply.

7. In apparatus for detecting defects in moving sheet materials havingsubstantially uniform reflection co-efficients, which senses, byphotoelectric means, variations in the intensity of light reflected fromthe surface of such a material as it passes over an inspection surface,a plurality of inspection units extending seriatim across said materialpassing over said inspection surface, each inspection unit comprising alower surface parallel and closely adjacent to the surface of saidmaterial when passing over said inspection surface, means defining anarrow elongated aperture in said lower surface extending transverselyto the direction of motion of said material and having a dimension insaid direction of the same order of magnitude as the diameter of thesmallest fully black defect to be detected, illumination meansilluminating that section of material beneath said aperture, a group ofphotoelectric cells positioned above and adjacent to said aperture andresponsive only to light from said illumination means reflected fromsaid section of material passing through said aperture, a constant gainamplifier wherein the gain is controlled only by ratios of resistors foramplifying the electrical output from said group of photoelectric cells,said group being connected to the input of said amplifier, the length ofsaid section viewed by said group at right angles to said direction ofmotion, being substantially greater than the diameter of the averagetexture discontinuity of said sheet material, the said sectionsassociated with said inspection units together forming a continuous lineextending across substantially the entire width of said material passingover said inspection surface, and every said constant gain amplifierbeing of equal gain and response.

8. In apparatus for detecting defects in moving sheet materials havingsubstantially uniform reflection characteristics, which senses, byphotoelectric means, variations in the intensity of light reflected fromthe surface of such a material as it passes over an inspection surface,an inspection head having a lower surface parallel and closely adjacentto the surface of said material when passing over said inspectionsurface, means defining a narrow elongated aperture in said lowersurface extending transversely to the direction of motion of saidmaterial and having a dimension in said direction of the same order ofmagnitude as the diameter of the smallest fully lack defect to bedetected, illumination means illuminating that section of materialbeneath said aperture, a group of photoelectric cells positioned aboveand adjacent to said aperture and responsive only to light from saidillumination means reflected from said section of material passingthrough said aperture, an amplifier for amplifying the electrical outputfrom said group of photoelectric cells, said group being connected tothe input of said amplifier, utilization means connected to the outputof said amplifier and responsive only to electrical signals of greateramplitude than that of the background noise in said group, afteramplification by said amplifier, and an integrator, connected betweenthe output of said group of cells, and the input of said utilizationmeans, having a charging time constant corresponding to the time oftravel beneath said inspection head of a predetermined distance in saiddirection of motion, whereby a barely perceptible defect of dimension insaid direction of motion equal to said distance and of dimension atright :angles to said direction of motion insufficient to produce achange in the magnitude of the electrical out put [from said group ofcells greater than the magnitude of said background noise level,produces an integrated output signal of sufficient amplitude to actuatesaid utilization means.

9. Apparatus according to claim 8 wherein said integrator is reactivelycoupled to the output of said group of cells, whereby said backgroundnoise is presented to said integrator as a signal of alternatingpolarity whose integrated output is low and substantially constant.

10. In apparatus for detecting defects in moving sheet materials, whichsenses, by photoelectric means, variations in the intensity of lightreflected from the surface of such a material as it passes over aninspection surface, an improved inspection head having a first surfaceparallel and closely adjacent to said inspection surface, means defininga first narrow elongated aperture in said first surface extendingtransversely to the direction of motion of said sheet material over saidinspection surface, illumination means uniformly illuminating thatsection of material passing beneath said first aperture, photoelectricmeans positioned above said first aperture and responsive only to lightfrom said illumination means reflected from said section passing throughsaid first aperture, and a sec ond surface parallel to said firstsurface, positioned intermediate said first surface and saidphotoelectric means, means defining a second narrow elongated aperturein said second surface parallel to said first aperture and ofsubstantially identical dimensions to said first aperture, the distancebetween said first and second surfaces being one order of magnitudegreater than the dimension of said first aperture in the direction ofmotion of said material over said inspection surface, and shieldingmeans having an inner surface of low co-efficient of reflection,shielding said second aperture from substantially all light except thatpassing through said first aperture, whereby to render saidphotoelectric means responsive only to light reflected from said sectionof material passing through both of said apertures.

11. Apparatus according to claim wherein said first and second aperturesboth lie in a plane perpendicular to the plane tangential to thatportion of said inspection surface immediately adjacent said firstaperture, and said illumination means comprises identical first andsecond lamp means positioned respectively on each side of said firstaperture and illuminating said section of material uniformly and with anequal intensity of light from each side of said aperture.

12. In apparatus for detecting defects in moving sheet materials, whichsenses, by photoelectric means, variations in the intensity of lightreflected from the surface of such a material when passing over aninspection surface, an inspection head having a lower surface paralleland closely adjacent to said inspection surface, means defining a narrowelongated aperture in said lower surface extending at right angles tothe direction of motion of said sheet material when passing over saidinspection surface, illumination means illuminating that section of saidmaterial passing beneath said aperture over an inspection surface, agroup of photoelectric cells positioned above said aperture andresponsive to light from said illumination means reflected from saidsection of material through said aperture, said group of cells beingconnected to a common set of terminals whereby to present across saidterminals a change in electrical output proportional to both theintensity and dimension at right angles to said direction of motion of adefect passing beneath said inspection head, and an integrator connectedto said common set of terminals for integrating said change inelectrical output, said integrator having a time constant not less thanthe time of travel beneath said head of a distance in said directionofmotion corresponding to the dimension in said direction of motion ofthe defect to be detected, said integrator giving an electrical outputsignal proportional to the product of the area and the intensity of adefect passing beneath said group of cells.

13. In apparatus for detecting defects in moving sheet materials ofsubstantially uniform reflection characteristics which senses byphotoelectric means variations in the intensity of light reflected fromthe surface of such a material when passing over an inspection surface,an

inspection head having a lower surface parallel, and closely adjacent,to the surface of said material when passing over said inspectionsurface, a plurality of photoelectric cells positioned above said lowersurface and adjacent thereto, each such cell viewing an associatedsection of said material surface through a slit-like aperture formed insaid lower surface, said aperture extending at right angles to thedirection of motion of said sheet material and having a dimension in thedirection of motion ofsaid sheet material of the same order of magnitudeas the diameter of the smallest fully black hzation circuit comprises apulse generator responsive only to signals of predetermined polarityabove a pre determined level, and voltage limiting circuit elementsdefect to be detected by said apparatus, lamps illuminating each saidassociated section, shielding means shielding each said photoelectriccell from substantially all except light from said lamps reflected fromits associated section through said aperture, at least one balancedpush-pull type amplifier having two balanceable input electrodes eachconnected to an equal number of photoelectric cells, and two outputelectrodes, and an utilization circuit connected across said outputelectrodes, whereby in phase and equal amplitude signals appliedsimultaneously to each input electrode produce no output signal acrosssaid utilization circuit.

14. Apparatus according .to claim 13 which includes at least two of saidamplifiers, and the photoelectric cells connected to the inputelectrodes of said amplifiers are so selected that no two photocellsresponsive to light reflected from contiguous associated sections areconnected to either of the input electrodes of the same amplifier.

15. Apparatus according to claim 13 which includes means for balancingthe direct current signal fed to each said input electrode by said equalnumbers of photoelectric cells when a sheet of defect free material ispositioned on said inspection surface beneath said inspection head.

16. Apparatus according to claim 13 wherein said amplifier includessignal inversion means for producing, from a defect signal of eitherpositive or negative polarity, an inverted defect signal of oppositephase to said defect signal, said defect signal being amplified by oneside of said push-pull amplifier and presented as a first output signalat one of said output electrodes, said inverted defect signal beingequally amplified by the other :side of said amplifier and presented asa second output signal of equal amplitude and opposite phase to saidfirst output signal, and said utilization circuit comprises a pulsegenerator responsive only to signals of a predetermined polarity, havingits input connected to both said output electrodes, whereby a defectsignal of either polarity produces an output signal of saidpredeterminedpolarity at the input to said pulse generator;

v 17. Apparatus according to claim 16, wherein said signal inversionmeans comprises a cathode coupled longtailed pair push-pull amplifyingstage.

I 18. Apparatus according to claim 13 wherein said amplifier comprises aplurality of pairs of amplifying stages connected in push-pull, directcurrent coupled in series, at least one of said pairs having a directcurrent feedback link to another one of said pairs earlier in saidseries, and at least one reactive circuit element decoupling alternatingcurrent components of signals in said feedback link, whereby to rendersaid amplifier as an alternating current amplifier having a frequencyresponse lying between upper and lower frequency limits definedrespectively by the time of travel beneath said inspection head of theshortest and longest defects in said direction of motion to be detectedby said apparatus.

19. Apparatus according to claim 13 wherein said utilization circuitcomprises a pulse generator responsive only to signals of predeterminedpolarity above a predetermined level, and voltage gate circuit elementsconnecting said pulse generator .to said output electrodes passing onlysigaals of said predetermined polarity above said predetermined level.

20. Apparatus according to claim 13 wherein said utilimiting amplifiedsignals appearing at said output electrodes to a value greater than saidpredetermined level and less than that at which said amplifier isblocked by said amplified signal. V

' 21. Apparatus according to claim 13 wherein said utilization circuitcomprises a pulse generator responsive only to signals above apredetermined level connected to said output electrodes, an integratorconnected between said output electrodes and said pulse generator, saidintegrator having a time constant corresponding to the time of travelbeneath said inspection head of a faint defect of dimension at rightangles to said direction of motion and intensity insufiicient to producean amplified defect signal at said output electrodes greater than saidpredetermined level, such a faint defect producing in said integrator anintegrated output signal greater than said predetermined level.

22. Apparatus according to claim 21 which further includes a resettingpulse generator connected to said integrator whereby to periodicallyreset said integrator.

23. Apparatus according to claim 15 wherein said pulse generator isresponsive only to signals of said predetermined polarity above apredetermined level and is connected to said output electrodes by firstvoltage gate circuit elements isolating said pulse generator fromsignals appearing at said output electrodes not of said predeterminedpolarity and above said predetermined level, and said utilizationcircuit further includes an integrator having its input connected tosaid output electrodes by second voltage gate circuit elements passingsignals only of the opposite polarity to said predetermined polarity,and having its output connected to said pulse generator by a voltagegate circuit element passing only signals of said predetermined polarityabove said predetermined level.

24. Apparatus according to claim 23 wherein said integrator is aMiller-type integrator, said opposite polarity is negative, the controlelectrode of said integrator being connected to said second voltage gatecircuit elements via a capacitance and a high resistance in series, andto the output of a resetting pulse generator via an isolation diode, thesteady state output level of said resetting pulse generator being morenegative than the potential needed at said control electrode to cut offsaid integrator, said resetting pulse generator periodically emittingpositive pulses of suiiicient amplitude to pass said isolation diode andreset said integrator.

25. Apparatus according to claim 24 which further includes a source ofvariable positive potential connected to one electrode of saidMiller-type integrator whereby to vary the said cut-off level of saidintegrator.

26. Apparatus according to claim 13 which includes a plurality ofidentical balanced push-pull amplifiers, and said utilization circuitscomprises a pulse generator having its input connected to a likeplurality of cathode follower stages, each cathode follower stage beingconnected to the output electrodes of a respective one of said pluralityof amplifiers.

27. Apparatus according to claim 23, wherein said second voltage gatecircuit elements are being connected through a capacitor to the input ofsaid integrator, the input of the integrator also being connectedthrough a resistor to a constant potential level, whereby backgroundnoise signals of alternating polarity present at said output electrodesare being halved and rectified and after a time interval, proportionalto the time constant of said capacitor and resistor, are decreased tosubstantially zero level at the input of said integrator.

28. Apparatus according to claim 1 wherein said inspection surface is adark surface having a low reflection coefficient whereby the detectionof transparent inclusions in thick paper is improved.

29. Apparatus according to claim 1 wherein said inspection surface has ahigh coeflicient of reflection whereby to minimize the signal variationsproduced in said photoelectric means by the lack of uniformity oftexture in unblemished sheets of said materials due to varyingtransparency.

30. Apparatus according to claim 1 wherein a plurality ofinterchangeable inspection surfaces having different colours andcoefficients of reflection are provided.

31. Apparatus according to claim 4 wherein said photoelectric meansconsists of a group of photoelectric cells, the number of cells in saidgroup being not more than that required to maintain the amplitude of thebackground noise level produced in said group below the change inamplitude of the electrical output of said group when a fully blackdefect of the smallest size to be detected passes beneath a cell of saidgroup.

32. Apparatus according to claim 4 wherein said photo electric meansconsists of a group of photoelectric cells and said aperture is of sucha length that the length of said section of material at right angles tosaid direction of motion is suflicient for a barely perceptible defectextending at right angles to the direction of motion a distance not lessthan said length of said section, to produce a change in the electricaloutput of said group of photoelectric cells of comparable magnitude tothat produced when a fully black defect of the smallest size to bedetected passes beneath a cell of said group.

33. Apparatus according to claim 4 wherein said photoelectric meansconsists of a group of photoelectric cells, and the length of saidsection viewed by said group at right angles to said direction of motionis substantially greater than the diameter of the average texturediscontinuity of said sheet material.

References Cited in the file of this patent UNITED STATES PATENTS1,828,000 Ranger Oct. 20, 1931 1,945,395 Cockrell Jan. 30, 19342,054,320 Hanson Sept. 15, 1936 2,078,800 Juchter Apr. 17, 19372,336,376 Tandler et al. Dec. 7, 1943 2,395,181 Hags Feb. 19, 19462,433,685 Dowell Dec. 30, 1947 2,438,588 Tolson Mar. 30, 1948 2,458,926Bassett Jan. 11, 1949 2,565,727 Henderson Aug. 28, 1951 2,617,528 MooreNov. 11, 1952 2,758,712 Linderman Aug. 14, 1956 2,939,016 Cannon May 31,1960 2,966,264 Cox Dec. 27, 1960

13. IN APPARATUS FOR DETECTING DEFECTS IN MOVING SHEET MATERIALS OFSUBSTANTIALLY UNIFORM REFLECTION CHARACTERISTICS WHICH SENSES BYPHOTOELECTRIC MEANS VARIATIONS IN THE INTENSITY OF LIGHT REFLECTED FROMTHE SURFACE OF SUCH A MATERIAL WHEN PASSING OVER AN INSPECTION SURFACE,AN INSPECTION HEAD HAVING A LOWER SURFACE PARALLEL, AND CLOSELYADJACENT, TO THE SURFACE OF SAID MATERIAL WHEN PASSING OVER SAIDINSPECTION SURFACE, A PLURALITY OF PHOTOELECTRIC CELLS POSITIONED ABOVESAID LOWER SURFACE AND ADJACENT THERETO, EACH SUCH CELL VIEWING ANASSOCIATED SECTION OF SAID MATERIAL SURFACE THROUGH A SLIT-LIKE APERTUREFORMED IN SAID LOWER SURFACE, SAID APERTURE EXTENDING AT RIGHT ANGLES TOTHE DIRECTION OF MOTION OF SAID SHEET MATERIAL AND HAVING A DIMENSION INTHE DIRECTION OF MOTION OF SAID SHEET MATERIAL OF THE SAME ORDER OFMAGNITUDE AS THE DIAMETER OF THE SMALLEST FULLY BLACK DEFECT TO BEDETECTED BY SAID APPARATUS, LAMPS ILLUMINATING EACH SAID ASSOCIATEDSECTION, SHIELDING MEANS SHIELDING EACH SAID PHOTOELECTRIC CELL FROMSUBSTANTIALLY ALL EXCEPT LIGHT FROM SAID LAMPS REFLECTED FROM ITSASSOCIATED SECTION THROUGH SAID APERTURE, AT LEAST ONE BALANCEDPUSH-PULL TYPE AMPLIFIER HAVING TWO BALANCEABLE INPUT ELECTRODES EACHCONNECTED TO AN EQUAL NUMBER OF PHOTOELECTRIC CELLS, AND TWO OUTPUTELECTRODES, AND AN UTILIZATION CIRCUIT CONNECTED ACROSS SAID OUTPUTELECTRODES, WHEREBY IN PHASE AND EQUAL AMPLITUDE SIGNALS APPLIEDSIMULTANEOUSLY TO EACH INPUT ELECTRODE PRODUCE NO OUTPUT SIGNAL ACROSSSAID UTILIZATION CIRCUIT.